As mentioned in U.S. Pat. No. 7,304,535, a vacuum tube balanced audio power amplifier normally contains three amplifying stages, namely an input stage, a second stage and an output stage. These are shown in FIGS. 3, 4, 5A, 5B, 6A, 6B, 9 and 10 of U.S. Pat. No. 7,304,535. Differential amplifiers of various kinds can be used to form the input and second stages. A commonly used vacuum tube differential amplifier is shown in FIG. 7 hereof. This differential amplifier can be used to form the first two stages of a conventional vacuum tube balanced audio power amplifier, as depicted in FIG. 8 hereof.
It is clear from FIG. 8 hereof that the input stage is a differential amplifier constructed by two triodes in the exact same form as shown in FIG. 7. Resistors R1 and R2 connect the grid to ground so that correct DC biasing can be set up for the tubes T1a and T1b. The outputs taken from the plates of vacuum tubes T1a, T1b are directly coupled respectively to the grids of vacuum tubes T5a, T5b of the second stage via series resistors R36 and R37. The second stage is also a differential amplifier in the exact same form. The outputs taken from the plates of the vacuum tubes T5a and T5b of the second stage are R-C coupled to the output stage via capacitors C10, C11 and resistors R25 and R26. The output stage consists of a pair of beam power tubes T3, T4 and output-matching transformer OPT.
Unlike small signal solid-state semiconductor transistors, in which the DC current gain (i.e., hFE, or sometimes referred to as DC amplifying factor) is very close for the same type of transistors, the DC amplifying factor of small signal vacuum tubes of the same type can differ immensely. It is because the advanced technologies of solid-state semiconductor fabrication allow the parameters of transistors to be tightly controlled. However, manufacturing of vacuum tube still relies on the skills of production workers in winding and aligning the wires and metal plates.
The differences in vacuum tube DC amplifying factor make it very difficult for vacuum tube amplifying stages to be directly coupled while maintaining the correct DC biasing for subsequent stages. It is obvious that in direct coupling of amplifying stages, the mismatched DC biasing voltages created from the first stage will be passed to the subsequent stages so that an even bigger mismatch of DC biasing voltages is created. As a result, the two vacuum tubes of a differential amplifier in a subsequent stage have to face two very different DC biasing voltages. Therefore, this leads to different biasing currents, signal voltage swings, output impedances and distortion levels at the two outputs of the differential amplifier in the subsequent stage. Hence the desired balancing properties of a vacuum tube balanced audio power amplifier can no longer be maintained.
In such a scenario, a common practice is to choose well-matched vacuum tubes for direct coupling applications. However, it is sometimes impractical and time consuming to screen the tubes. Even if well-matched vacuum tubes are used, as the tubes get aged after a period of operation, the mismatch will eventually resurface as some tubes deteriorate faster than others. Therefore, tube aging again creates mismatch of DC biasing. Another conventional method is to use R-C coupling rather than direct coupling between amplifying stages. This can totally eliminate the mismatched DC biasing problem passing from previous stages as DC voltages are completely blocked by the coupling capacitors. However, it is well known that an amplifier formed by R-C coupling of amplifying stages generally has poorer low frequency response than one formed by direct coupling. Poor low frequency response is not desirable in audio application.
The aim of this invention is to provide a new differential amplifier such that the use of well-matched vacuum tubes is no longer an absolute necessity for direct coupling applications. This invention will at least reduce the DC biasing mismatch to an acceptable level for most direct coupling applications. In addition, a grid-to-cathode over-voltage protection is included.
According to the present invention, there is provided a single stage differential amplifier including a pair of vacuum tube triodes for amplifying two input signals and generating two output signals, wherein said input signals are fed to the grids of said pair of vacuum tube triodes, and a pair of two series resistors on each grid is cross-connected to two separate junctions formed by a pair of two series resistors, respectively, such that the latter pair of series resistors are connected together with a constant current source connected to a negative power supply.